In an IMS network, each SIP User Agent (UA) is provisioned in the Home Subscriber Server (HSS), and the User Agent must first register with the network before making and receiving calls. In one example, the SIP User Agents are individual user endpoints. In contrast, an Aggregate Endpoint (AEP) is an endpoint that has multiple subtending users, but appears as a single SIP User Agent to the IMS network. Most of the endpoints supported by existing Voice over Internet Protocol (VoIP) services for enterprise applications, e.g., an Internet Protocol Private Branch eXchange (IP PBX), have multiple users behind it and may not perform standard SIP based IMS registrations.
Unlike single user endpoints, calls will always be allowed to or from the IP PBX. However, unregistered users behind an IP PBX are unable to utilize services provided by an IMS network.